The present invention relates to a method and a device for stabilizing a vehicle equipped with a slip-controlled brake system.
A stabilization device for passenger vehicles is described in the article xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d, published in the automotive engineering journal ATZ Automobiltechnische Zeitschrift, vol. 96, no. 11 (1994) pp. 674-689. With this stabilization device, the yaw rate and float angle of the vehicle are regulated by the control system. This is done by determining a control deviation between the actual values and the setpoint values for the float angle and the yaw rate. Starting with these setpoints, changes in setpoint slip which are implemented through appropriate engine control measures and/or brake control measures are determined. With the help of brake control measures, it is possible to increase or reduce the brake pressure on individual wheels according to the changes in setpoint slip.
A system for brake slip control is described in the article xe2x80x9cAntiblockiersystem (ABS) f{dot over (u)}r Personenkraftwagenxe2x80x9d, published in xe2x80x9cBosch Technische Berichtexe2x80x9d, volume 7, (1980, no. 2, ISSN 006-789 X). In this system, the actual slip is compared with a setpoint slip during a braking operation, and as soon as the actual slip is equal to the setpoint slip, the brake pressure on the respective wheel is reduced.
For example, German Published Patent Application No. 2 112 669 describes an anti-lock device which controls wheel slip in such a way that maximum braking forces between the road surface and the tire are possible. This anti-lock device has a control device with which the anti-lock protection can be influenced when the longitudinal axis of the vehicle deviates from the direction of travel. This control device includes a final controlling element which automatically reduces wheel slip to a value at which lateral guidance of the wheel is improved significantly with hardly any noticeable reduction in brake pressure. The final controlling element reacts to the direction in which the vehicle travels along a curve. Among other things, this anti-lock device causes the braking force on the inside of the curve to be reduced and the braking force on the outside of the curve to be increased.
All the devices described above as belonging to the related art use wheel slip as a controlled variable to implement measures influencing the wheel.
The present invention provides a method and a device using an alternative controlled variable other than wheel slip so that measures to influence the wheels can be implemented on the basis of this alternative controlled variable.
The present invention provides a method for stabilizing a vehicle equipped with a slip-controlled brake system. In this method, wheel speeds are determined for the individual wheels of the vehicle. As an alternative to the slip, as a controlled variable, the wheel speed is used in the method according to the present invention. For this reason, a setpoint value for the wheel speed is determined for at least one wheel. A deviation quantity describing the deviation between the wheel speed determined for this wheel, and the setpoint value is determined for the at least one wheel. To stabilize the vehicle, the actuators assigned to the at least one wheel are controlled as a function of this deviation quantity.
Slip is a relative quantity; in other words, if slip is controlled by activating the actuators assigned to the wheel, then the wheel speed that will result after activating the actuators is not always known in advance. Wheel speed, however, is an absolute quantity. If wheel speed is regulated by activating the actuators assigned to the wheel, then the wheel speed which results after activating the actuators can be set at a desired level. Thus when traveling along curves, for example, the wheel speed resulting after activating the actuators can be adapted to the actual curve turned.
The method and device according to the present invention are to be used in vehicles equipped with an anti-lock control unit. This is not a restriction on the scope. Use in vehicles equipped with traction control or a form of slip control such as that described in the above-mentioned ATZ article is also possible.
Vehicles may also tend to oversteer during turning even in the partial braking range. If a vehicle is equipped with a brake slip control system, it can be stabilized with the help of the method according to the present invention. To stabilize the vehicle, pressure-modulating control measures which are not brake slip control measures per se can be implemented on the front axle.
In addition, due to the load reduction on the front wheel on the inside of the curve, vehicles equipped with a brake slip control system can be brought under brake slip control with this wheel very quickly. This can be perceptible in the pedal itself and from noises which constitute an impairment of driving comfort. The method according to the present invention can also prevent the front wheel on the inside of the curve from coming under brake slip control with even slight braking due to the load reduction at higher transverse accelerations. With the method according to the present invention, an improvement in both stability and comfort is achieved with brake slip control systems.
The slip-controlled brake system used with the device according to the present invention is one in which at least the brake slip of individual wheels of a vehicle can be controlled. The vehicle is in the partial braking range during this stabilization.
The at least one wheel whose actuators are activated as a function of the deviation quantity can be a wheel on the inside of the curve. In a further embodiment, it can be desired that the at least one wheel is the front wheel on the inside of the curve. The actuators are activated so that the braking effect prevailing on the wheel on the inside of the curve is reduced in comparison with the braking effect prevailing on the wheel on the same axle but on the outside of the curve. The following is achieved with the method of influencing the front wheel on the inside of the curve as described above. Since the method according to the present invention is designed primarily for a vehicle equipped with a slip-controlled brake system, only a control measure that reduces the braking effect and is to be implemented independently of the driver may be considered as a stabilizing control measure on the actuators assigned to the wheels. An increase in braking effect may not happen because such brake systems that operate by hydraulic or pneumatic means do not have any means for increasing pressure independently of the driver so that ultimately an increase in braking effect can be achieved. With the method according to the present invention, the tendency of the vehicle to oversteer is to be counteracted, so a yaw moment with an understeering effect must be produced. Since only a reduction in braking effect can be implemented independently of the driver, as mentioned above, it must be implemented on the wheels on the inside of the curve in order to achieve a yaw moment with an understeering effect. The front wheel on the inside of the curve is selected because due to the load distribution the transmission of force to this wheel on the inside of the curve can be more favorable than that to the rear wheel on the inside of the curve.
The setpoint for wheel speed as a function of the wheel speed of the wheel on the outside of the curve which is on the same axle as the wheel on the inside of the curve is used advantageously for the wheel speed. This type of determination is performed because the wheel on the outside of the curve is especially stable when traveling along curves because of the load distribution of the vehicle. It is likewise advantageous if the difference in wheel speeds which results when traveling along curves for wheels on the inside and outside of the curve is taken into account in determining the setpoint. This can be accomplished by determining the setpoint as a function of a quantity describing the radius of the curve being turned.
An embodiment of the present invention further provides a two-step procedure for determination of the setpoint. First, an intermediate quantity is determined as a function of the wheel speed of the wheel on the outside of the curve on the same axle as the wheel on the inside of the curve and/or as a function of a quantity describing the radius of the curve being turned. This intermediate quantity describes the wheel speed expected for the wheel on the inside of the curve due to traveling along a curve. The radius of the curve being turned is determined as a function of the vehicle speed and the transverse acceleration acting on the vehicle. In other words, the intermediate quantity thus determined represents the speed of the wheel on the inside of the curve which this wheel must theoretically have for a certain curve radius and a certain transverse acceleration if cornering effects are to be taken into account. Even if the wheel on the outside of the curve does have some slip in comparison with the vehicle speed, determining the intermediate quantity in this way can still be used. In this case, the wheel on the inside of the curve and the wheel on the outside of the curve are certain to have equal amounts of slip. In other words, there is no difference in slip between the two wheels. In a second step, the intermediate quantity is weighted with a predetermined factor to determine the setpoint. This weighting should be done in such a manner that the setpoint is reduced by a predetermined proportion in comparison with the intermediate quantity due to this weighting. This achieves the result that the wheel speed set on the wheel on the inside of the curve will correspond to the given conditions due to cornering in any case. The smaller the predetermined proportion by which the intermediate quantity is reduced, the more braking force is transmitted by a the front wheel on the inside of the curve.
The deviation quantity is the difference, based on the setpoint, between the wheel speed determined for the at least one wheel and the setpoint. This deviation quantity has the nature of slip due to its determination. This deviation quantity can be filtered, which is done by using a PT1 element known from the related art. The actuators are activated as a function of the filtered deviation quantity. Filtering the deviation quantity ensures that the noise of the speed signals does not have an interfering effect. In addition, short-term disturbances from the road surface are also damped.
The deviation quantity is compared with upper and lower threshold values. The actuators are then activated as a function of this comparison, so that the deviation quantity remains within the value range defined by the upper and lower threshold values and/or is guided into this range.
The actuators can be activated so that the braking effect prevailing on the at least one wheel is reduced when the deviation quantity is lower than the lower threshold value and/or it is retained when the deviation quantity is greater than the lower threshold value and less than the upper threshold value and/or it is increased when the deviation quantity is greater than the upper threshold value. In this way, the deviation quantity is kept within the value range described above.
In activating the actuators, a check is performed to determine whether a condition, according to which an increase in braking effect on the at least one wheel is to be suppressed, is met, and in the case when this condition is met, instead of increasing the braking effect, it is kept constant. This procedure is based on the following state of affairs. If actuators assigned to the front wheel on the inside of the curve are activated according to the present invention to stabilize the vehicle, there is an increase in load on the wheel on the inside of the curve as the vehicle travels more slowly and the radius of the curve remains the same. This leads to a reduction in slip on this wheel. Therefore, the actuators may be activated in such a way that a braking effect is produced. For example, if it is a hydraulic brake system, there will be an increase in pressure on this wheel. These increases in pressure are not absolutely necessary at a low speed and low deceleration, but they have a negative effect on pedal comfort due to the buildup pulse series until the admission pressure level is reached. To suppress this negative effect on pedal comfort, a pressure buildup is suppressed when such a situation is detected and instead the pressure and thus the braking effect are kept constant.
According to a further embodiment of the present invention, the setpoint and/or deviation quantity is determined when a corresponding conformity condition is met and/or activation of the actuators of the at least one wheel is released when a corresponding release condition is met and/or activation of the actuators of the at least one wheel is terminated when a corresponding termination condition is met.